Hmmm! If you have the time, I suggest you try only a few slides through one or two of the services. I remember having 35 mm color slides made from computer files about 15 years ago. The originals were just graphs and charts for a technical presentation in full color. The service made them at a resolution of 3000 lines - that is, I believe, 3000 lines to cover the top-to-bottom space. On a slide that's about one inch, and 3000 lines per inch sounds great. But put that into a projector and blow it up to 5 feet high (make that 60 inches), and we're looking at 50 lines per inch. My recollection is that the slides looked just fine for the presentation, but I was not trying to reproduce fine detail and subtle shading. And people then were not used to seeing the fine video we expect now.
Your last post mentions "my portfolio", making me think you are trying to show detailed artwork of some kind. That's why I suggest you check with a few slides whether the services can do what you need. They would have to provide high resolution (eg 3000 lines vertically or more) plus highly detailed color reproduction (at least 24-bit color).